It's likely that Naruto himself could be very powerful. His ability to use chakra and perform powerful jutsus like the Rasengan and Sage Mode would give him an edge. He could also use his shadow clones to confuse opponents.
Sasuke would also be a strong contender. With his Sharingan, he can copy his opponents' moves and his Rinnegan gives him access to even more powerful abilities like Amenotejikara. His Chidori is a fast and deadly attack that could catch opponents off guard in the Smash Bros arena.
However, you can't discount characters like Kirby in this context. Kirby has the ability to copy the powers of his opponents in the Super Smash Bros games. If he were to face Naruto characters, he might be able to copy their jutsu and use it against them. Also, his floating ability and quick attacks make him a formidable foe. So, it really depends on how the author of the fanfic wants to balance the powers and abilities of the different characters.
It could be Naruto in his Nine - Tails Chakra Mode. He has immense power and speed, which would give him an edge in the Smash Bros battles.
It's likely Goku. His Super Saiyan forms give him incredible power, speed, and energy attacks. He can constantly increase his power level and has a lot of combat experience from his battles in the Dragon Ball series.
However, Saitama from One Punch Man (if we were to include him in this crossover fanfiction for the sake of comparison) would be extremely powerful. His one - punch ability means he can defeat almost any opponent with a single strike. But in the context of My Hero Academia characters only, Todoroki might also be a strong contender. His ability to control ice and fire gives him a great range of offensive and defensive capabilities in the battles.
Naruto and Kirby would be really interesting. Kirby could copy Naruto's abilities, like his running speed or his ability to use chakra. And Naruto could be amazed at Kirby's cuteness and his unique way of fighting, like inhaling enemies. They could have a really fun dynamic together.
Goku would probably be the most overpowered. His ability to constantly increase his power level and his wide range of energy attacks like the Kamehameha would make him a tough opponent in Super Smash Bros. He can also fly, which gives him an advantage in the game's arenas.
Goku would likely be one of the most powerful. His ability to constantly increase his power level through training and his Super Saiyan forms give him an edge. He can also use his ki attacks at a very high level. However, Kirby could also be a strong contender. He has the ability to copy the powers of his opponents. If he were to copy Goku's power, for example, he could become extremely powerful. Another powerful character could be Meta Knight. His swordsmanship and flight abilities combined with his strategic mind would make him a formidable foe in this crossover.
Mario is often a very popular character. He's such a well - known icon from the gaming world, and in fan fiction, he can be involved in all kinds of adventures. He might be on a quest to rescue Princess Peach from a new villain in the Smash Bros world. Link is also popular. His sword - fighting skills and his mysterious backstory make him a great subject for fan - created stories. Writers can explore his connection to the Triforce and how it affects his role in the Smash Bros battles.
Mario is often a popular choice. He's such an iconic character that people love to write about his adventures in the Smash Bros world. His relationship with other characters, like Luigi or Peach, can be explored in many ways.
There's also the idea that a character like Luigi, who is often overlooked but has his own unique skills, could become very powerful when combined with Naruto's abilities. Imagine Luigi using the invisibility power - up along with Naruto's stealth skills. He could be a formidable opponent in this fanfiction crossover.